The present invention relates to power transmission, and, in particular, a speed variator for providing continuously variable ratio on the transmission of power.
Many approaches have been taken in the prior art to provide for transmission of power between a prime mover, such as an engine or motor, and a device driven thereby. In the vehicular field, transmissions are available providing a variety of gear ratios, manually or automatically. However, there has been continuing interest in power transmitting to provide stepless, continuously variable speed transmission wherein the relative speed between an input shaft and an output shaft can be continuously varied.
A favored approach has been to utilize a variator system in which the speed of an output shaft is dependent on the relative speeds of the input shaft and an auxiliary variator shaft. Utilizing primarily variable pitch pulleys, planetary gearing and differential gearing, varying components have been used to allow an operator to achieve an output ratio in accordance with operator preference or equipment performance demands.
In U.S. Pat. No. 5,167,591 to Cowan, an infinitely adjustable, variable speed transmission for bicycles is disclosed wherein a variator including manually adjustable variable pitch pulleys are used to vary the output speed of planetary gearing connected to the driven bicycle wheel. A related system is disclosed in U.S. Pat. No. 5,167,591 to Cowan. A further such system is disclosed in U.S. Pat. No. 4,553,450 to Gizard. The speed ratio range afforded the pulley system is limited.
Other approaches have used toroidal race drives such as disclosed in U.S. Pat. Nos. 5,667,458 to Fellows and 4,628,766 to Perry. Such transmissions are mechanically complex and costly. Electromechanical braking of one shaft of a twin differential gear set is disclosed in U.S. Pat. No. 2,441,606 to Trofimov to provide limited variation in gear ratio between a prime mover and an output drive train.
In view of the foregoing, it would be desirable to provide a compact variator for infinitely adjusting the output speed in a drive train in a manner that overcomes the foregoing limitations heretofore associated with variable pitch pulleys and friction drives, and like variators.
Accordingly, it is an object of the present invention to provide a variator providing affirmative, continuously adjustable differential output speeds.
Another object of the invention is to provide a variable ratio transmission using a hydraulic brake to continuously vary output speed between ratio limits.
A further object of the invention is to provide a speed variator for a drive transmission using a variable flow, positive displacement hydraulic circuit to vary the output speed of a planetary drive train.
The foregoing objects are accomplished by a variable speed transition wherein a secondary differential disposed between the prime mover input shaft and differential driven output shafts has one output shaft coupled with a variable flow control valve and positive displacement pump in a hydraulic circuit for providing variable braking thereto effecting a ratio change in the other output shaft thereby selectively changing the drive train ratio. The secondary differential may employ planetary gearing or conventional differential bevel gearing. At open condition, the controlled output shaft is minimally braked and a direct drive is provided to the output differential. As the valve is closed, the output pressure increase at the positive displacement pump and effects an increasing resistive torque at the shaft resulting in an increased speed at the output shaft. At full control valve closure, the differential is locked and a final drive ratio approaching the theoretic ratio of 2:1 is established, with continuous variable control therebetween. For incorporation in to vehicles, from passenger and recreational to commercial, appropriate clutching and reverse gearing may be incorporated.